For many years, I just bought tomato plants when I was ready to plant them. When purchasing them locally, that limited me to a tiny number of choices. By starting my plants from seed, it gives me a vast array of options, and it gives me much more control over timelines. I start my seeds in peat pellets. The Jiffy peat pellets are what I use. They come dry, and you just need to add warm water to them, and they will expand. Here is how it should look.
I take a toothpick and open up the peat to a depth of 1/2 inch. Then I place two seeds into each pellet and cover them by pulling the peat from the outside back over into the middle. I also like to add a sprinkle of water to eliminate any gaps down to the seeds. After a week or so, you will begin to see sprouts popping up. Here’s an example of mine after one week.
These first two leaves are called seed leaves or cotyledons. They are part of the embryo of the plant, and you may even see them with the seed husk still attached. These leaves will not have the typical leaf shape of a tomato, but those leaves will come soon enough. Notice how the stem of the plant is leaning heavily to one side. This is typical indoors, and the plant appears to be reaching for the light. At this point, I would orient the plant, so it is directly under the light source. I lower my grow light down to around 6 inches above the top of the plant. It is not always possible, but it is also a good idea to get your plant outside for some amount of time. This will strengthen the plant by exposing it to full sunlight and wind.
After another week, you will see the real leaves begin to appear. You may also notice roots poking through the outer mesh. You need to be diligent about keeping the pellets wet at this time since the pellets are relatively small. Otherwise, I will leave the plants alone until they are at least 6 inches tall. At this point, you are ready to move them to a bigger pot. I use the cheap plastic nursery pots that are 6 inches in diameter at the top and approximately 4 1/2 inches deep. The first thing you must do is move the mesh covering from the peat. Some folks say it isn’t necessary, but I have accidentally left it on and found it fully intact at the end of the growing season. After you have removed the mesh, place the plant in the bottom of the pot. Pinch off any leaves that are below the top of the container. Then fill the pot with whatever type of soil or planting mix you prefer. It should look something like this.
This would be fine to plant in your garden. If I planted it at this stage, I would leave the soil level as it is. If the plant is larger than this, plant it deep enough that approximately this much of the plant remains above ground, removing any branches below the soil line before planting. When you do this with a tomato plant, roots will form along the stem underground and further support and nourish your plant.